1. Field of the Invention
The invention relates to a proximity detector for a body scanner having a scanning surface. The body scanner may for example be an x-ray computed tomography system. It may also be an ultrasound body scanner. In particular this invention relates to a proximity detector for an Anger-type scintillation gamma camera which is adapted for emission computerized tomography (ECT).
2. Description of Prior Art
U.S. Pat. No. 4,158,776 (Barret) described a patient sensing and indicating arrangement for a computed tomography system, wherein the patient has to be centered within a reconstruction circle of a rotating cylindric body scanner. The arrangement comprises a set of three light emitting sources projecting light beams in a plane which is perpendicular to both the cylindric scanning surface of the body scanner and the cranial-caudal axis of the patient. The attachment also comprises three light sensors for the projected light beams. Three independent light beams generally define three sides of a square having sides of a certain length. When the arrangement rotates these three light beams define a circle having a diameter according to the side lengths of the square. This circle is the reconstruction circle in which the patient has to be centered. Due to decentering of the patient any of the light beams will be interrupted. Thus, a signal can be produced to indicate that the patient is not in a correct central position.
In the case of U.S. Pat. No. 4,158,776 the cylindric body scanner always rotates around an axis which has a fixed distance with respect to the scanning surface of the body scanner. To make this fixed axis coincident with the patient's cranial-caudal axis the patient has to be centered. However, very often, particularly in ECT scintillation gamma camera scanning, scanning surface of the body scanner is not fixed with respect to a central axis. In contrast to U.S. Pat. No. 4,158,776 it is desired to make the distance between scanning surface of the body scanner and the patient's body variable, as to adapt rotation of the body scanner around the patient's body as closely as possible to the body structure. The proximity detector as described in U.S. Pat. No. 4,158,776 is no longer useful. A typical ECT scintillation gamma camera is for example taught in U.S. Pat. No. 4,417,143.